Photodiode for ultra high speed optical communication and fabrication method therefor

ABSTRACT

Disclosed is a photodiode having a p-type electrode of a mushroom shape. The p-type electrode is formed in a mushroom shape, so that the contact area faced by the spreading region of a dopant for the photodiode and the electrode can be minimized and the capacitance of the photodiode can be reduced. Further, the p-type electrode is configured to have a broader width in its upper end, thus allowing the wire bonding to be performed easily.

CLAIM OF PRIORITY

[0001] This application claims priority to an application entitled“PHOTODIODE FOR ULTRA HIGH SPEED OPTICAL COMMUNICATION AND FABRICATIONMETHOD THEREFOR,” filed in the Korean Industrial Property Office on Jan.8, 2002 and assigned Serial No. 02-959, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical receiving element,and particularly to a photodiode used in an optical communicationsystem.

[0004] 2. Description of the Related Art

[0005] In the optical communication, an electrical signal is convertedinto an optical signal at the transmitting end using a light emittingelement and then transmitted through a transmission line, such as anoptical fiber. The converted optical signal is converted back into anelectrical signal at the receiving end using a light receiving element,such as a photodiode. Most widely used photodiodes have a mesa typestructure.

[0006]FIG. 1 is a cross-sectional view of a conventional photodiode 10having a mesa type structure. As shown in FIG. 1, the non-doped InGaAsand p-InP are sequentially stacked at one end of an InP substrate 11 bymeans of a single crystal growing process. An u-InGaAs absorption layer12 of a mesa type and a p-InP window layer 13 are formed by etching.Thereafter, silicon nitride(SiNx) is stacked on the p-InP window layer13 so that an insulation layer 14 is formed, and a predetermined portionof the insulation layer 14 is etched so that a part of the p-InP windowlayer 13 has an opening. A p-type electrode 15 is provided on the openportion of the p-InP window layer 13. Meanwhile, a non-reflectioncoating is applied to the position corresponding to the p-InP windowlayer 13 on the other side of the InP substrate 11, so that a lightreceiving region 17 having a predetermined size and an n-type electrode16 are formed.

[0007] In the method of fabricating a photodiode of a mesa type asdescribed above, the non-doped InGaAs and p-InP layers are stackedthrough the single crystal growth process. Further, the undesirablespreading of a p-type dopant, such as Zn or Cd, is not necessary.

[0008] The conventional photodiode having the mesa type structure,however, has several drawbacks in that the non-doped InGaAs and thep-InP are formed as layers in the form of a mesa type, then exposed tothe atmosphere. As such, the non-doped InGaAs and the p-InP materialscan be oxidized during a fabrication process. This oxidation may cause adeterioration in the quality of the optical element. In addition, acurrent leakage may occur on the surface that is mesa-etched, i.e., thesurface facing the insulation layer, thereby reducing the life of theoptical component. Moreover, the InGaAs film whose energy band gap issmall, may have a larger current leakage, thus further deteriorating thereliability of the optical circuit.

[0009] Furthermore, in an ultra high-speed optical communication, thecapacitance of the optical circuit must be small which can be achievedby reducing the spreading region of the p-InP. The photodiode of arelated art, however, makes wire bonding by providing a p-type electrodeconnected directly to the spreading region of the p-InP. Therefore, itis difficult to reduce the area of the spreading region of the p-InP.

SUMMARY OF THE INVENTION

[0010] The present invention overcomes the above-described problems, andprovides additional advantages, by providing a photodiode used in anultra high-speed optical communication and its fabrication methodcapable of improving reliability and lowering the capacitance thereof,by preventing a current leakage and enabling easier subsequent wirebonding.

[0011] According to one aspect of the invention, a photodiode for ultrahigh speed optical communication includes: a substrate; an absorptionlayer formed on an upper surface of the substrate; a window layerstacked on the absorption layer; an insulation layer stacked on thewindow layer and having a predetermined hole formed thereon; a spreadingregion in which a predetermined dopant is spread on the part of thewindow layer facing the hole; and, an upper electrode connected to thespreading region through the hole and formed in a mushroom shape abovethe insulation layer.

[0012] According to another aspect of the invention, a photodiode forultra high speed optical communication includes: an InP substrate; anInGaAs absorption layer stacked on one side of the InP substrate; an InPwindow layer stacked on the InGaAs absorption layer; an insulation layerstacked on the InP window layer and having a predetermined hole formedthereon; a p-InP spreading region in which a p-type dopant is doped onthe part of the InP window layer facing the hole; a metal of electricalconductivity electrically connected to the p-InP spreading regionthrough the hole and formed in a mushroom shape above the insulationlayer.

[0013] According to a further aspect of the invention, a method forfabricating a photodiode for ultra high speed optical communicationincludes the steps of: preparing a predetermined semiconductor substratein which a spreading region is included, wherein an absorption layer, awindow layer, and an insulation layer are stacked in sequence on thepredetermined substrate, and the spreading region is formed by spreadinga predetermined dopant on the window layer through an opening formed onthe part of the insulation layer; forming a photoresist layer on theinsulation layer; forming a hole on the photoresist layer by means of aphotoetching method so that the hole is connected to the opening of theinsulation layer; forming a metal plated layer connected to thespreading region through the hole formed on the photoresist layer; and,removing the photoresist layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will be described in detail with reference to thefollowing drawing in which like reference numerals refer to likeelements wherein:

[0015]FIG. 1 is a cross-sectional view illustrating a general photodiodeof a mesa type;

[0016]FIG. 2 is a cross-sectional view illustrating a photodiode forultra high speed optical communication according to a preferredembodiment of the present invention;

[0017]FIG. 3 is a photograph illustrating a metal of electricalconductivity as shown in FIG. 2, formed on a photodiode; and,

[0018]FIG. 4a through FIG. 4e are illustrative drawings showing theprocess of forming the metal of electrical conductivity shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] The following detailed description will present a photodiode forultra high speed optical communication and fabrication method thereofaccording to a preferred embodiment of the invention with reference tothe accompanying drawings. For the purposes of clarity and simplicity,well-known functions or constructions are not described in detail asthey would obscure the invention in unnecessary detail.

[0020]FIG. 2 is a cross-sectional view illustrating a photodiode 20 usedin an ultra high-speed optical communication system according to apreferred embodiment of the present invention, and FIG. 3 shows aphotograph of electrical conductivity of the electrode formed on thephotodiode 20 shown in FIG. 2. As shown in FIG. 2, the inventivephotodiode 20 includes a u-InGaAs absorption layer 22 on which anon-doped InGaAs is stacked, is formed entirely on at one end of an InPsubstrate 21; an u-InP window layer 23 a on which a non-doped InP isstacked, is formed on the u-InGaAs absorption layer 22; and, aninsulation layer 24 a on which a SiNx is stacked, is formed on the u-InPwindow layer 23 a.

[0021] A predetermined hole 24 b is formed on the insulation layer 24 a,and a p-InP spreading region 23 b on which a p-type dopant such as Zn isdoped, is formed on a predetermined region of the u-InP window layer 23a facing the through hole 24 b. The p-InP spreading region 23 b isextended up to a portion of the u-InGaAs absorption layer 22. The p-InPspreading region 23 b is connected to a p-type electrode 25 a providedthrough the hole 24 b. The p-type electrode 25 a connected to the p-InPspreading region 23 b through the hole 24 b is exposed above theinsulation layer 24 a and is formed in a mushroom shape above theinsulation layer 24 a. Namely, the height of the p-type electrode 25 adoes not coincide with the surface of the insulation layer 24 a, but isextended further above the insulation layer 24 a. The area at which thep-type electrode 25 a and the surface of the insulation layer 24 a faceeach other is narrower than the surface area of the p-InP spreadingregion 23 b and shaped, so that the farthest away the p-type electrode25 a is distant from the surface of the insulation layer 24 a, thebroader the p-type electrode 25 a becomes. As a result, a predeterminedspace is formed between the outer periphery of the p-type electrode 25 aand the insulation layer 24 a, whereby an air layer 25 b is obtained.Note that if the p-type electrode 25 a is broad enough, the process ofwire bonding is easier to implement.

[0022] Meanwhile, on the other side of the InP substrate 21, anon-reflection coating using a silicon nitride(referred as SiNxhereinafter) is performed on the position corresponding to the p-InPspreading region 23 b, so that a light receiving region 27 is formed.For the rest region, except for the light receiving region 27, thenon-reflection coating is removed, then an n-type electrode 26 using anelectrical conductivity metal is formed, thereby forming a photodiode20.

[0023] Note that the development of the optical communication was adirect result of higher data transmission speed, and that thetransmission speed in the optical communication largely depends on thetransmission speed of an optical communication element, such as thephotodiode 20. The photodiode 20 used in an ultra high-speed opticalcommunication can improve the transmission speed by increasing thetransmission bandwidth. To this end, the smaller the resistance and thecapacitance of an optical element are, an increase in the transmissionbandwidth can be achieved. As most optical elements today need to be ascompact as possible, the capacitance rather than the resistance plays amajor role on the transmission bandwidth in the optical communicationsystems, as noted below mathematically.

[0024] The capacitance of the element for optical communication is givenby the following formula 1:

[0025] [Formula 1] $C = \frac{ɛ\quad A}{t}$

[0026] Here, A represents an active area activated as the photodiodereceives an optical signal, and ε and t represent a dielectric constantof material occupying the space between electrodes and the distancebetween electrodes, respectively.

[0027] According to the formula 1, if the active area A becomes narrowand the dielectric constant ε becomes smaller and the distance t betweenthe electrodes becomes larger, then the capacitance C gets smaller.Therefore, in order to make the capacitance of the photodiode 20smaller, the p-InP spreading region 23 b, an active area, must beminimized and the dielectric constant c must be reduced. To meet theseconditions, the photodiode 20 according to the present invention reducesthe p-InP spreading region 23 b by minimizing the contact area of thep-type electrode 25 a and the p-InP spreading region 23 b. This resultis achieved by having the p-type electrode 25 a formed in a shape suchthat the farther away the p-type electrode 25 a away from the surfacefacing the p-InP spreading region 23 b, the broader the p-type electrode25 a becomes in its width. As a result, the space between the surface ofthe insulation layer 24 a and the p-type electrode 25 a is filled withair of low dielectric constant, whereby an air layer 25 b is formed.Further, the air layer 25 b giving the shape of the p-type electrode 25a lowers the dielectric constant E substantially.

[0028] As apparent from the foregoing, the present invention minimizesthe contact area of the p-type electrode 25 a and the p-InP spreadingregion 23 b, thereby reducing the area for the active region of theoptical element used in the optical communication. Therefore, thephotodiode of the present invention is easy to apply to the ultra highspeed optical communication system and has sufficient space for the wirebonding through the formation of the p-type electrode 25 a in a mushroomshape.

[0029] Now, the process for forming the p-type electrode having amushroom shape will be described hereinafter.

[0030]FIG. 4a and FIG. 4b depict the procedures of forming a photoresistlayer on a semiconductor substrate 40. As shown in FIG. 4a, thesemiconductor substrate 40 includes a predetermined substrate 41, anabsorption layer 42, a window layer 43 a in which a spreading region 43b is included therein, and an insulation layer 44 a on which a hole 44 bis formed. Further, a photoresist is applied on the semiconductorsubstrate 40 so that a photoresist layer 45 a is formed as shown in FIG.4b.

[0031] Thereafter, the hole 45 b is formed on the photoresist layer 45 awith use of the photolithography method as shown in FIG. 4c. The hole 45b is formed in a cylindrical shape by photoetching the hole 44 b on theinsulation layer 44 a, then the process for forming the sidewalls isperformed, so that the hole 45 b has a broader shape in its upperportion. The hole 45 b is connected to the hole 44 b of the insulationlayer 44 a and shaped so that the width of the hole 45 b is broader asit moves farther away from the region facing the insulation layer 44 a.

[0032]FIG. 4d shows that the metal 46 of electrical conductivity havinga mushroom shape is formed. The metal 46 of electrical conductivity isformed in a mushroom shape as the shape of the hole 45 b on thephotoresist layer 45 a becomes broader as it moves farther away from theinsulation layer 44 a. The metal 46 of electrical conductivity is formedby plating and serves as the p-type electrode, which is positionedhigher than the photoresist layer 45 a. As the spreading region 43 b ismade of semiconductor material, a predetermined metal layer (not shown)is deposited and formed on the window layer 43 a for a subsequentplating to be performed.

[0033] After the metal 46 of electrical conductivity is formed, thephotoresist layer 45 a is removed as shown in FIG. 4e, thennon-reflection coating is applied on a rear side of the substrate 41,and the coating for the rest region, except for the region correspondingto the spreading region 43 b, is removed by means of a photoetchingprocess, thus forming a light receiving region 47 a. An n-type electrode47 b is formed on the region where the non-reflection coating is removedusing another metal of electrical conductivity, thereby completing theformation of a photodiode.

[0034] As described above, the present invention forms the p-typeelectrode of the photodiode in a mushroom shape, thereby minimizingcapacitance of the photodiode, while accomplishing the electrode of astructure in which the wire bonding is easy to apply. That is, in thephotodiode of a related art, there have been limitations in reducing theactive area to secure the area for performing the wire bonding on thep-type electrode. The photodiode according to the present invention,however, forms the p-type electrode of a mushroom shape, therebyminimizing the area facing the p-InP spreading region as well assecuring the area for the wire bonding.

[0035] In addition, the photodiode of the present invention lowerscapacitance by reducing the area for the p-InP spreading region. Also,the etching process is not performed for the u-InGaAs absorption layeror the u-InP window layer stacked on the InP substrate, instead, amethod of spreading a p-type dopant on the u-InP window layer isadopted, thus preventing the u-InGaAs absorption layer or the u-InPwindow layer from being exposed to the atmosphere and oxidizedaccordingly. Moreover, the contact area of the u-InGaAs absorption layerwith the insulation layer is blocked by the u-InP window layer, so thatthe u-InGaAs absorption layer is not exposed to the current leakage,thereby improving the reliability of the optical elements. Furthermore,the p-type electrode is formed in a mushroom shape, whereby a sufficientbonding area is secured which in turn makes the wired bonding easy.

[0036] The foregoing embodiments and advantages are merely exemplary andare not to be construed as limiting the present invention. Thedescription of the present invention is intended to be illustrative, notto limit the scope of the claims. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. In the claims,means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also an equivalent structure.

What is claimed is:
 1. A photodiode for ultra high speed opticalcommunication comprising: a substrate; an absorption layer formed on anupper surface of the substrate; a window layer stacked on the absorptionlayer; an insulation layer having a predetermined hole stacked on thewindow layer; a spreading region having a predetermined dopant spread ona part of the window layer facing the predetermined hole; and, anelectrode having a mushroom shape disposed above the insulation layerand connected to one end of the spreading region through thepredetermined hole.
 2. The photodiode according to claim 1, wherein theother end of the spreading region is further extended up to a portion ofthe absorption layer.
 3. The photodiode according to claim 1, whereinthe electrode comprises an upper surface and a lower surface, the widthof the upper surface of the electrode is substantially broader the lowersurface of the electrode, so that a space between the insulation layerand an outer periphery of the electrode forms a contact area for asubsequent wire boding.
 4. The photodiode according to claim 3, whereinthe space between the insulation layer and an outer periphery of theelectrode is filled with air of low dielectric constant.
 5. Thephotodiode according to claim 1, wherein a contact area between theupper electrode and the insulation layer is narrower than the surfacearea of the spreading region.
 6. A photodiode for ultra high speedoptical communication comprising: an InP substrate; an InGaAs absorptionlayer stacked on at one end of the InP substrate; an InP window layerstacked on the InGaAs absorption layer; an insulation layer having apredetermined hole stacked on the InP window layer; a p-InP spreadingregion in which a p-type dopant is doped on a part of the InP windowlayer facing the hole; and, a metal conducting element having a mushroomshape above the insulation layer electrically connected to the p-InPspreading region through the predetermined hole.
 7. The photodiodeaccording to claim 6, wherein the other end of the p-InP spreadingregion is further extended up to a part of the InGaAs absorption layer.8. The photodiode according to claim 6, wherein the electrode comprisesan upper surface and a lower surface, the width of the upper surface ofthe electrode is substantially broader than the lower surface of theelectrode, so that a space between the insulation layer and an outerperiphery of the metal conducting element forms a contact area for asubsequent wire boding.
 9. The photodiode according to claim 6, whereinthe space between the insulation layer and an outer periphery of theelectrode is filled with air of low dielectric constant.
 10. Thephotodiode according to claim 6, wherein a contact area mutually facedby the metal conducting element and the insulation layer is narrowerthan the surface area of the spreading region.
 11. A method forfabricating a photodiode used in an ultra high-speed opticalcommunication system, the method comprising the steps of: providing anabsorption layer, a window layer, and an insulation layer stacked insequence on a semiconductor substrate; spreading of a predetermineddopant amount on the window layer through an opening of the insulationlayer to form a spreading region; forming a photoresist layer on theinsulation layer; forming a hole on the photoresist layer by means of aphotoetching method so that the hole is connected to the opening of theinsulation layer; forming a metal-plated layer connected to thespreading region through the hole formed on the photoresist layer; and,removing the remanent photoresist layer.
 12. The method according toclaim 11, further comprising the step of processing an inner sidewall ofthe photoresist layer so that the hole formed on the photoresist layerbecomes broader in its width gradually in an upward direction.
 13. Themethod according to claim 11, wherein the metal plated layer ispositioned higher than the photoresist layer.
 14. The method accordingto claim 11, wherein the metal plated layer comprises a mushroom shape.15. The method according to claim 11, wherein the step of removing thephotoresist layer is performed by a photo-etching technique.